1. Field of the Invention
The present invention relates to a Voice Call Continuity (VCC), and more particularly, to requesting a domain transfer in the VCC.
2. Discussion of the Background Art
In general, a Voice Call Continuity (VCC) refers to a type of application, namely, a home IMS (IP Multimedia Subsystem) application which is capable of transporting voice calls between a CS (Circuit Switching) domain and an IMS domain. The VCC provides functions of voice call originations, voice call terminations, a domain selection and a domain transfer from the CS domain to the IMS domain or vice versa. Here, the domain transfer refers to transferring access legs for voice calls toward a user equipment (UE) (i.e., a terminal) from the CS domain to the IMS domain or vice versa during an active session. The access leg denotes a call control leg between a VCC UE and a Domain Transfer Function (DTF) of an VCC application (server).
Through the domain transfer procedures, a service continuity for one or more voice calls/sessions is provided between the IMS domain and the CS domain while the VCC UE maintains the one or more voice calls/sessions.
Generally, a domain transfer for a certain voice call/session from the CS domain to the IMS domain or vice versa is initiated only when a DTF is positioned on a signal path of the voice call/session setup. For this, positioning of the DTF on the way of the signal path of the voice call/session setup is referred to as anchoring in IMS or anchoring.
FIG. 1 illustrates a general architecture of a network for providing a VCC service.
As illustrated in FIG. 1, a VCC UE 10 denotes all types of terminals which support the VCC service. The VCC UE can access CS and PS (packet switching) domains. That is, when accessing the CS domain, the VCC UE uses a UE-CS (not shown) provided therein, whereas the VCC UE uses a UE-IMS (not shown) provided therein when accessing the PS domain.
A VCC application 30 is an application server for providing the VCC service, and is constituted with entities which perform a series of functions. The series of functions may include functions required to setup voice calls toward the VCC UE, and functions required to switch an access leg of the VCC UE between the CS domain and the IMS domain while maintaining (performing) an active session. For instance, the series of functions can be a Domain Transfer Function 30a, a Domain Selection Function 30d, a CS Adaptation Function 30b, and a CAMEL Service Application 30c. Detailed capabilities and operations for the series of functions are described in 3GPP TS 23.206 V1.2.0.
Generally, the CS domain entities include a Visited Mobile Switching Center (VMSC), a Gateway MSC (GMSC), a gsmSCF, and the like. The IMS domain entities include a P-CSCF, a S-CSCF, a I-CSCF and a Media Gateway Control Function (MGCF).
FIG. 2 is a signal flowchart among each component of a network (e.g., the network shown in FIG. 1) in a case where a domain transfer occurs between the IMS domain and CS domain. Hereinafter, domain transfer procedures in a VCC service according to a background art will be explained with reference to FIG. 2.
As shown in FIG. 2, it is just a VCC UE which initiates a domain transfer in a VCC. That is, the VCC UE establishes a voice call (or session) over the CS domain or IMS domain with a network, and then may decide whether or not to initiate a domain transfer by moving (or transferring) from one domain to another domain. By doing this, the established voice call (so-called “ongoing call”) may continue on the way of handing over (i.e., domain transfer) from the CS domain to the IMS domain or vice verse. Here, for initiating (performing) a domain transfer for the voice call from one domain (e.g., IMS domain) to another domain (e.g., CS domain), the VCC UE may initiate the domain transfer based on previously-stored information. Here, the information previously stored in the VCC UE may include radio conditions of a CS network to access, operator policy, user preferences, and the like.
Procedures (1)˜(4) of initiating, by the VCC UE, a domain transfer from a CS domain to an IMS domain according to the background art are as follows:                (1) The VCC UE may decide to initiate a domain transfer for a voice call which has been originated and is ongoing toward the CS domain, namely, an outgoing call.        (2) When the VCC UE sends an INVITE message to a VCC application, a Domain Transfer Function (DTF) in the VCC application establishes an IMS session leg for the voice call over the IMS domain.        (3) After the IMS session leg is established between the DTF and the IMS domain, the DTF routs the ongoing session to the IMS domain.        (4) After routing the ongoing session to the IMS domain, a CS session leg is released.        
Thus, the domain transfer for the voice call is initiated from the CS domain to the IMS domain through the procedures of (1)˜(4).
For the domain transfer in the VCC, only the VCC UE can initiate the domain transfer in the current VCC by taking into account certain criteria (e.g., user preferences, operator policies, etc.). Therefore, according to the background art, there is no way for the network (e.g., VCC application) to initiate a domain transfer based upon a current network state, a data load of a specific node, a sustainable QoS (Quality of Service), or the location of the VCC UE.
Further, the network may want to reduce traffic or stop the flow of traffic over a specific domain during a certain time duration for maintenance or upgrading of the network. In this case, according to the background art, there is no method for the network to allow (or request) the VCC UE to initiate the domain transfer for a current call over a specific domain (i.e., over the CS domain or IMS domain).